1. Technical Field
The invention relates to an intrusion detector comprising a passive sensor for detecting a person entering a space to be monitored, said intrusion detector comprising a housing provided with a window for said passive sensor, optical means for directing electromagnetic radiation from said person onto the passive sensor, alarm means connected to said passive sensor for generating an alarm in case the electromagnetic radiation from said person being detected by the passive sensor corresponds to a signal value that exceeds a maximum level or falls below a minimum level.
2. Background
Such an intrusion detector is known from the present Applicant's International (PCT) patent publication No. 2006/107203. Disposed behind the window of the housing of the known intrusion detector is a passive infrared sensor in the form of a pyro-electrical sensor which is sensitive to infrared light in the far infrared wavelength range. When a burglar, for example, enters the space to be monitored, infrared light (with a wavelength of 6-50 μm) emitted by the burglar (on account of the burglar's body heat) will be detected by the pyro-electrical sensor functioning as a passive infrared sensor, and subsequently an alarm signal will be generated. The intrusion detector, through its pyro-electrical sensor, thus functions as a motion detector. To prevent the intrusion detector being sabotaged while in its state of rest, for example when the pyro-electrical sensor is deactivated during the daytime, for example as a result of lacquer or paint being sprayed on the window or of the intrusion detector as a whole being covered with a hat, a coat or the like, the known intrusion detector is configured with a so-called “anti-masking” or “anti-sabotage” system. Said system thus functions to protect the intrusion detector generally against sabotage attempts, in particular against being approached, masked or damaged. Such “anti-masking” systems generally comprise a light source and a light detector optically coupled thereto, which “monitor” the direct vicinity of the housing as well as the window. When a person approaches the housing and/or the window therein, this will lead to a significant increase or decrease (viz. diffusion/reflection or absorption of emitted light by the person) of the light being detected by the light detector and consequently to an alarm signal being generated.
U.S. Pat. No. 4,375,034 (Guscott) likewise discloses a passive infrared intrusion detection system disposed in a space to be monitored. The intrusion detection system disclosed therein comprises a focussing mirror and a cylindrical mirror which cooperates therewith to form a protective curtain in the space to be monitored, with a pyro-electrical sensor being disposed on the optical axis at the focus of the focussing mirror. When an intruder moves through the protective curtain—in the operative condition of the intrusion detection system—the focussing mirror will focus infrared light emitted by the burglar (on account of the latter's body heat) onto the pyro-electrical sensor via the cylindrical mirror, which will detect said infrared light. Detection signals corresponding thereto are subsequently electronically processed in order to produce an alarm output to signal the presence of the intruder.
A drawback of the use of several protective curtains as described in the aforesaid U.S. Pat. No. 4,375,034 in passive infrared intrusion detectors known from the aforesaid International (PCT) patent publication No. 2006/107203 is that only a limited number of protective curtains can be formed, whilst in practice there is a growing need for a higher monitoring density, i.e. a larger number of protective curtains. After all, the passive sensor, for example a pyro-electrical sensor which is sensitive to infrared light, has a limited aperture angle, which limits the number of focusing mirrors to be used. Moreover, the use of a larger number of smaller focussing mirrors does not offer a solution, since this would have a disproportionate adverse effect on the desired signal/noise ratio.